Rolling begins with preheated sheet ingots that can weigh more than 20 tons. As the size of rolling mills has increased, so has the size of these ingots, but a typical ingot is approximately 6 feet wide, 20 feet long and more than 2 feet thick. The ingot is first heated to rolling temperature and fed into a breakdown mill, where it is rolled back and forth until the thickness has been reduced to just a few inches. The slab can be subsequently cold rolled or may be heat-treated to increase its strength. The highest strength alloys are heat treated and rapidly cooled to room temperature, after which they are stretched to straighten and relieve internal stress built up during rolling and heat-treating. They are aged naturally at room temperature or artificially aged in a furnace to develop the desired combination of strength and corrosion resistance. Finally, the plate is trimmed to final size. Plates produced in this manner may be used at full thickness, but are often machined into a variety of simple to complex shapes.